Opaque chrome coating has been used for many years as a low reflectance, opaque aperture coating for optical elements, photomasks, and black matrix for LCD displays. Opaque chrome coating typically has three layers: a very thin chrome (Cr) flash for adhesion to a substrate, followed by a chrome oxide (CrOx) layer for low reflection, followed by a thicker chrome (Cr) layer for opacity. The thickness and composition of the opaque chrome layers are chosen to achieve a desired opacity and low reflectance. Optimal layer composition and thickness may be experimentally determined or derived (P. Baumeister, “Starting designs for the computer optimization of optical coatings,” Appl. Opt. 34(22) 4835 (1995)). Carbon and nitrogen are often added to improve the reflectance and etch resistance of some of the layers (e.g., U.S. Pat. No. 5,230,971 issued to Alpay). More complex opaque chrome coating structures are known (e.g., U.S. Pat. No. 5,976,639 issued to Iwata).
Opaque chrome coating layers are usually deposited on a substrate by a physical vapor deposition, typically thermal evaporation, e.g., electron beam evaporation or resistance evaporation, or sputtering. An aperture can be patterned in the opaque chrome coating layers with standard photolithography, either by lift-off or etch-back. Lift-off involves depositing the opaque chrome layers over the patterned resist. Stripping the photoresist in a suitable solvent removes the chrome on top the resist to form the pattern. A less critical structure can easily be patterned by lift-off. However, lift-off is less suitable for applications which require patterns with straight edges. More critical structures are typically patterned by etch-back where resist is patterned over the opaque chrome coating layers. The pattern is transferred to the opaque chrome layers by wet etching, typically with a solution of perchloric acid and cerium ammonium nitrate, or by dry etching with a chlorine and oxygen plasma. Recently, this dry etching process has been adopted in the photomask industry because it permits etching of finer features than wet etching. Dry etching of chrome has been discussed extensively in the literature (Y. Huang et al., “Extended chamber matching and repeatability study for chrome etch,” Proc. SPIE—Int. Soc. Opt. Eng., Vol. 4562, pp. 624-632 (2002), J. O. Clevenger et al., “Effect of chamber seasoning on the chrome dry etch process,” Proc. SPIE—Int. Soc. Opt. Eng. vol. 5130, no. 92-100 (2003), R. B. Anderson et al., “Study of the role of Cl2, O2, and He in the chrome etch process with optical emission spectroscopy,” Proc. SPIE—Int. Soc. Opt. Eng., vol. 4889, pp. 641-652 (2002), R. B. Anderson et al., “Improvement of chrome CDU by optimizing focus ring design,” Proc. SPIE—Int. Soc. Opt. Eng. vol. 5130, no. 1, pp. 264-174 (2003), and M. Mueller et al., “Dry etching of chrome for photomasks for 100 nm technology using chemically amplified resist,” Proc. SPIE—Int. Soc. Opt. Eng., Vol. 4754, pp. 350-360 (2002)).
One of the most economical methods for depositing opaque chrome coating layers on a substrate is ion-assisted electron beam evaporation. In general, the method involves sequentially generating vapors of chrome (Cr) and chrome oxide (CrOx) using an electron beam evaporator and depositing the vapors on a substrate while bombarding the film growing on the substrate with a low energy ion beam. The ion-assist allows for denser and more uniform films than without ion-assist. The more uniform the films, the more consistent the optical properties of the opaque chrome coating. The denser the films, the more resistant the opaque chrome coating is to cracking and pinhole formation. The ion-assist also minimizes the stress in the films. On the other hand, haze and stain of substrate in an aperture etched in opaque chrome coating layers deposited by ion-assisted electron beam evaporation have been observed. This haze and stain can affect the transmission and reflection properties of the aperture.